fixIO bug

[Stefan Ljungstrand]

The code is a small modification of mine to the attribute-grammar-like
(multi-pass lazy) example with recursive IO at
http://www.cse.ogi.edu/PacSoft/projects/rmb/repMin.html

The original code replaces every (leaf) elements in a tree by the minimum
of them all, returning the new tree, while printing out the elements, in
traversion order.

My modification tries to also print out minimum (at each non-leaf call)
the global minimum (which is lazily passed down as a value from the
'future'). Probably this should generate a "Fail: <<loop>>" at first
attempt to prematurely force the global minimum (or maybe earlier).

The problem is, it actually duplicates some of the other IO effects a few
times, before grinding to a "Fail: <<loop>>" stop !

The files :

---- IO.hs
import System.IO (fixIO)

data Tree a = L a | B (Tree a) (Tree a) deriving Show

-- One-pass, with IO on both passes
rpMin :: Tree Int ->  Int -> IO (Tree Int, Int)
rpMin (L a)   m = do putStr "Elem " ; print a
                     return (L m, a)
rpMin (B l r) m = do (l', ml) <- rpMin l m
                     (r', mr) <- rpMin r m
                     let m' = ml `min` mr
                     putStr "LocMin " ; print m'
                     putStr "GlobMin " ; print m    -- oops ! :)
                     return (B l' r', m')

replaceMin :: Tree Int -> IO (Tree Int)
replaceMin t = fmap fst $ fixIO (\ ~(t', m) -> rpMin t m)

main = replaceMin (B (L 1) (L 2))
---- linux-version
Linux page-208 2.4.22-gentoo-r7 #1 Wed Mar 3 02:12:36 PST 2004 i686 AMD Athlon(tm) XP 1800+ AuthenticAMD GNU/Linux
---- ghc-version
The Glorious Glasgow Haskell Compilation System, version 6.2.1
---- commands
ghc IO.hs > ghc-output 2>&1
./a.out > output 2>&1
---- ghc-output (see attachment)
---- output
Elem 1
Elem 2
LocMin 1
GlobMin Elem 1
Elem 2
LocMin 1
GlobMin Elem 1
Elem 2
LocMin 1
GlobMin Elem 1
Elem 2
LocMin 1
GlobMin Elem 1
Elem 2
LocMin 1
GlobMin Elem 1
Elem 2
LocMin 1
GlobMin Elem 1
Elem 2
LocMin 1
GlobMin Elem 1
Elem 2
LocMin 1
GlobMin Elem 1
Elem 2
LocMin 1
GlobMin Elem 1
Elem 2
LocMin 1
GlobMin
Fail: <<loop>>
----

(Note : The code in the attached file was not run in GHC by me,
        but by skew)

-- 
Stefan Lj
md9slj

The infinity that can be finitely expressed is not the true infinity
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